This nice cryptomachine was quite thoroughly described in the July 1989 issue of the excellent journal Cryptologia, and introduced with the following words:
The HC-9 is a post World War II mechanical ciphering machine manufactured by AB Transvertex in Sweden and probably used by the Swedish military forces up to the 1970's.
The machine was designed for relatively low level use; platoon, company, up to battalion levels and in regimental and brigade staffs...
It seems as if this is true, since my own HC-9, rescued from demounting after becoming obsolete, was delivered to me in an original case made of more than a centimetre thick layer of rubber, suggesting use under field conditions.
With some further citing, now from the AB Transvertex HC-9 Manual, the ciphering system will be described:
The HC-9 ciphering machine is designed on the ciphering principle that a drum with 16 incoherent (i.e. "mixed") alphabets rotates a certain number of stages (0-15) in relation to a fixed starting point (step 0) by the addition of four rotation-increments, representing 8 or 0, 4 or 0, 2 or 0, and 1 or 0 stages of rotation. These increments are formed by the fact that of 5 different revolutions (corresponding to pin wheels with 29, 31, 33, 34 and 35 segments) continuous revolutions are read and combined separately into "pairs": 29-31, 31-33, 33-34, 34-35. The five revolutions are read regularly one step at a time, and the total period will therefore be 29x31x33x34x35= 35,303,730 stages.
The active (1) or passive (0) impulses read from the revolutions are converted into rotation increments in a special adding mechanism in such a way that every "pair" is combined according to their symbols, on the principle that 11 and 00 give a rotation-increment of 8 for the pair 29-31, of 4 for the pair 31-33, of 2 for the pair 33-34, and of 1 for the pair 34-35, 10 and 01 always giving the rotation-increment 0. These rotation-increments are then added up to give the final rotation, which may consequently vary from 0 to 15 stages.
To operate the machine, one needs a punched card which contains the pinwheel data. The card is inserted into the back of
the machine, and a lid is closed. Further, one needs a sheet of paper with 16 mixed alphabets. These alphabets must be
reciprocal, i.e. if the first letter of one alphabet is, for instance, an E, the letter at position 5 (E is the
fifth letter of the alphabet) must be an A, if the second letter is a C then the third must be a B,
and so on.
The sheet containing the alphabets is rolled around a staff and locked into place with a little beam. Then the staff is inserted through a slit on the right side of the machine and a second lid is closed.
Now the machine is ready for cryptowork. To encipher a message, the operator first choses a five letter-group at random (or according to some other agreed upon scheme). On the top of the machine are situated five turnable wheels. On the surface of each wheel, the normal alphabet is visible through a window, and the wheels are turned until the five letters are set against an index also visible through the window.
Since each wheel has got more than 26 positions, some positions between the letter A and Z are left blank (and hence can not be indicated).
On the left hand side of the machine is a lever. By pushing this lever down, the mechanism is operated and one of the 16
alphabets on the sheet becomes visible through another window, this one situated on the front of the machine. Above this
window, the normal alphabet is printed and the cleartext letter is sought in this alphabet and substituted for the one
visible directly beneeth in the window (this is done by hand. The machine does not print the text.). Now the lever is pushed again and another (or sometimes the same) alphabet becomes visible through the window. The second cleartext letter is
substituted, and so the operator continues until all letters are enciphered.
Together with the cryptogram, the operator must transmit information giving the five letters chosen initially, so that the receiver can set up his machine accordingly. Since the alphabets on the sheet are reciprocal, the receiver can then, after operating the lever on his machine, look up the cryptoletter in the ordered alphabet and find the cleartextletter directly beneeth on the alphabet visible through the window.
If you like, you can download a Qbasic simulation of the HC-9 here and start encrypting your Internet communications in the real way, instead of using some boring PGP program, which hasn't got a clue as to what a five letter group is: